I've got a thumbdrive (SanDisk Cruzer Mini 128MB) and a Lexar JumpDrive Trio USB media reader. With both of them, they work flawlessly in Windows (95 and 98 require a driver, 2000/XP don't) and on OS 9. But on OS X, they're inconsistent.
Most frequently, I'll plug them into the USB ports (front or back) of the G5 towers at work, and they'll light up, but OS X 10.3.2 and 10.3.6 never recognizes or mounts them. I have to reboot the G5 with the drive plugged in for the Mac to recognize it, and then it refuses to eject the drive -- no error message, the drive just stays on the desktop forever, regardless of what I can do to try to dismount it (namely, cmd-e and dragging to the trash). I usually just pull the drive and get a warning that I'm supposed to eject them first.
Less frequently, the drive never apparently gets power at all -- the lights never come on. This led me to believe that the ports might be faulty, except that every G5 in this building and at my old college does it. Boot the same machines into 9.2.2 and the drives work flawlessly, every time.
Aside from this, I have a Creative Zen Micro. In removable disk mode -- which again works flawlessly under Windows and OS 9 without drivers -- the disk locks up while copying anything onto or from it. The Zen freezes in transfer mode, and Finder needs a hard restart.
What gives? I've Googled to find other people with this problem for weeks and haven't found anything. I've formatted the Cruzer as FAT32, and the memory card as FAT32, NTFS and HFS+. Same problem every time.